The properties of thin film on ASIC image sensors are discussed starting from the dynamic range and linearity of the a-Si:H detector element. Depending on the operation mode, the dynamic range of the detector exceeds the performance of conventional CCDs by far. Limitations arising from a non optimized operation mode are examined employing a semi- empirical simulation model. They can be overcome with pixel electronics adapted to the demands of the detector. In order to maximize the dynamic range of the complete array, different modes of signal transport from the pixels to the peripheral circuitry are investigated.